Electric fuse with indicating mechanism

ABSTRACT

A current-limiting power fuse is provided with an improved indicating mechanism. The mechanism includes a conductive support cup mounted on one terminal cap and containing a cup-shaped conductive indicator button in telescoping fashion, the bottom of the indicator button being remote from the bottom of the support cup, and a spring between the button and cup to exert an outward force on the button. An insulating bushing fitted into the bottom of the support cup has two apertures. A resistance wire passes through one aperture, through a securing point on the bottom of the button, and back through the other aperture of the bushing. The wire lengths extending outside from the apertures are pulled taut to restrain the button against the force of the spring and secured together outside the bushing after passing in opposite directions through the center of a bead resting on the bushing between the apertures. Then the lengths of the wire are led through the body of the fuse and connected to the opposite terminal cap.

United States Patent [1 1 Gray et al.

July 15, 1975 1 ELECTRIC FUSE WITH INDICATING MECHANISM [7 3] Assignee:

[22] Filed:

General Electric Company,

Pittsfield, Mass July 22, 1974 [21] Appl. No; 490,797

[52] US. Cl 337/244; 337/267 [51] Int. Cl. "01h 85/30 [58] Field of Search 337/241, 244, 79, 267,

References Cited UNITED STATES PATENTS 12/1942 6/1969 8/1970 l/1972 4/1972 l0/l973 Bicrmanns et a1. 337/244 Inglis 337/244 X Feenan et al. 337/244 X Cameron 337/244 Wilson 337/244 Swain et al. 337/244 Primary Examiner-J. D. Miller Assistant Examiner-Fred E. Bell Attorney, Agent, or FirmVolker R. Ulbrich 571 ABSTRACT A current-limiting power fuse is provided with an improved indicating mechanism. The mechanism includes a conductive support cup mounted on one terminal cap and containing a cup-shaped conductive indicator button in telescoping fashion, the bottom of the indicator button being remote from the bottom of the support cup, and a spring between the button and cup to exert an outward force on the button. An insulating bushing fitted into the bottom of the support cup has two apertures. A resistance wire passes through one aperture, through a securing point on the bottom of the button, and back through the other aperture of the bushing. The wire lengths extending outside from the apertures are pulled taut to restrain the button against the force of the spring and secured together outside the bushing after passing in opposite directions through the center of a bead resting on the bushing between the apertures. Then the lengths of the wire are led through the body of the fuse and con nected to the opposite terminal cap.

4 Claims, 4 Drawing Figures ELEL'TRIL FISF WITH INDICA'I'ING MPIHANISH IlAL'KGROt'NI) OF THF INVENTION The present invention relates generally to electrical fuses ha ing means for indicating a fused condition. and relates particularly to current-limiting power fuses with such indicating means.

Current-limiting power fuses generally ha\e a cylindrical housing of insulating material which is closed at each end by a metal terminal cap. A main fusible clement. usually a sil\ er ribbon. extends inside the housing between the terminals. The space around the fuse element is filled with silica sand. When current through the fuse element exceeds the rating ofthe fuse fora suf ficient time. the element melts. or fuses at one or more points. with the resulting formation of arcs there. The arcs progressnely melt the sand and form channels of fulgurite in it. The relatively high resistance fulgurite e\entually suppresses any significant How of current through the fuse.

Because the melting of current-limiting fuses of the type described above is silent and does not normally result in an obvious change in the outward appearance of the fuse. it is common to provide on the fuse an indicating means by which it can be readily determined visually that the fuse has melted. or fused. One such iridicating means is. for example. the button indicator mechanism described in LES. Pat. No. 2.797.279 issued June 25. 1957 to Brandt et al.. In this mechanism. an insulating support cup is mounted to the inside face of one of the terminal caps over an opening in the cap. A cup-shaped. insulating indicator button is situated in a telescoping fashion inside the support cup. A coil spring inside the button forces the button away from the bottom of the support cup. The button is held in place against the force of the spring by a restraining resistance wire which passes through an aperture in the bottom of the support cup. under an attachment tab protruding from the bottom of the button. and out through a second aperture in the bottom of the support cup. The wire is fixed under tension by tapered pins pressed into the apertures. The wire length extending from one of the apertures is electrically connected to the immediate terminal cap. while the length extending from the other aperture proceeds through the silica sand in the housing to connection at the other terminal cap.

When the main element of the fuse melts. the line oltage appears across the terminal caps. This results in melting of the restraining wire inside the button. The unrestrained button is now pushed out through the opening in the terminal cap by the spring. until a lip about its open end stops against the periphery of the opening. The protruding portion ofthe button provides a ready visual indication that the fuse has melted.

For some purposes it is desirable that the indicator not only give \isual indication of the condition of the fuse. but also actuate another mechanism. For example. where a number of large electric motors. each proided with a current-limiting fuse are drawing current from the different phases of a secondary circuit. it is often desirable to cause a circuit breaker to shut off all the motors on the line when one fails. This prevents damage to the other motors by uneven phase loading which might result from the disconnecting ofthe faulty motor from one of the phases by the fuse. This could be accomplished by utili/ing the motion of the indicat ing button to trip a circuit breaker. The force needed to reliably trip commonly used circuit breakers. hovw c\er. is on the order of several pounds. whereas the force exerted by the button of the above-described mechanism is only on the order of one-half pound. Moreover. modification of the mechanism to pro\ide the desired force involves certain difficulties.

One difficulty is that when the spring of the present mechanism is replaced with one exerting a force of sev cral pounds. the restraining wires are unable to hold the button against the force and slide in the apertures. despite the locking action of the pegs.

Another difficulty is that a spring force on the order of several pounds tends to deform the insulating material of the button. thus decreasing its reliability. While some insulating materials might be available which would sufficiently minimize such deformation. they are relatively costly as compared to the presently used phcnolic resin material. The substitution of metal for the phenolic resin. on the other hand. presents problems of electrical isolation of the restraining wire.

SUMMARY OF THE INVENTION The notel fuse is protided with an improved indicating mechanism. The mechanism comprises a support cup and indicator button which are in electrical contact with the terminal cap. A spring exerts a relatively large force between the button and cup. A restraining wire which passes through an apertured bushing plug in the bottom of the support eup needs connection outside the cup only to the other terminal.

Since the support cup and button are conductive. they can be made inexpensively of metal and can with stand a spring pressure on the order of several pounds without having their function impaired by deformation.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. I is a partially sectioned side view of an electrical power fuse provided with an indicating mechanism in accordance with the preferred embodiment of the present invention.

FIG. 2 is an enlarged. sectional side view of an end cap of the fuse of FIG. I. showing in more detail the in dicating mechanism.

FIG. 3 is an enlarged. sectional side view of an alter nate embodiment of the end cap and indicating mechanism for the fuse of FIG. 1.

FIG. 4 is a top view of the end cap and indicating mechanism of FIG. 3.

DESCRIPTION OF THE PREFERRED EMBODIMENTS One embodiment of the invention is the motor starter fuse 10 shown in FIG. I of the drawing and rated for about 600 amperes. The fuse 10 includes a tube T2 of fiberglass having an upper brass ferrule [4 at one end and a lower ferrule 16 at the other end. The end openings of the upper and lower ferrules I4. 16 are closed by upper and lower brass end caps 18. 20. respectively. Two fusible ribbon conductors 22 of silver. the main fuse elements. are strung along the inside length of the tube 12. passing through disc-shaped insulating spacers 24 at the ends. on the other side of which the ribbons 22 are connected under tension to the ferrules l4. lb. The remaining space around the ribbons 22 inside the tube 12 is filled with an interrupting medium of finely divided quartz sand 26.

To the upper end cap 18 is part of an indicating mechanism 28, shown in greater detail in the FIG. 2. A round metal support cup 30, formed by a short wall section 32 of brass tubing which is upset around a brass washer 34 to form a bottom, is brazed at its open end to the inside surface of a matching opening centrally located in the upper end cap 18. Inside the support cup 30, and oppositely oriented with respect to it, is a cupshaped metal indicator button 36. The button 36 is telescoped inside the support cup 30 with sufficient clearance to permit free upward movement and has an outwardly extending lip 38, which in conjunction with three evenly spaced convex projections 40 in the wall 32 of the support cup 30 limits the upward travel of the button 36. An eyelet 42 in the central portion of the bottom inside the button 36 serves as an attachment point for wire.

inside the button 36 is a stainless steel coil compression spring 44 which exerts a biasing force of about two pounds between the bottom of the button 36 and the bottom of the support cup 30. Fitted into a central opening in the washer 34, which forms the bottom of the support cup 30, is an insulating bushing 46 of phe nolic resin having a flange 48 which rests on the washer 34 about the perimeter of the opening. Two round apertures 50 spaced oppositely from the center of the bushing 46 pass through it to the interior of the support cup 30.

A nickel-chromium restraining wire 52 having a diameter of thousandths of an inch and a composition of essentially 80 percent nickel and percent chromium, by atomic weight, passes through one of the apertures 50 into the button 36, is looped through the eyelet 42 and passes back out through the other aperture 50. The wire 52 is pulled tight so that the button lip 38 rests on the bottom washer 34 of the support cup against the force of the spring 44. The wire 52 is fastened under tension by having its ends passed in opposite directions through a perforation in a cylindrical phenolic resin bead 54 lying transversely on the outside of the bushing 46 between the apertures 50. After being passed through the perforation, the extending ends of the wire 52 are tied firmly over the bead 54 in a square knot and twisted. The free ends of the wire 52 pass through the sand 26 to the opposite lower end cap 20 and are secured there between the lower end cap 20 and the lower ferrule 16 where these two members are joined, so that electrical contact is established between the ferrule 16 and the wire 52.

When an overload condition is present in the fuse 10, causing the fuse ribbons 22 to melt, the sections of the wire 52 holding the button 36 against the force of the spring 44 also melt, thereby releasing the button 36 outward until its lip 38 is stopped by the projections 40. The movement of the button 36 may be utilized to activate a circuit breaker and also provides a clear visual indication that the fuse 10 has melted.

Since in the fuse 10 the support cup 30 and button 36 are both of metal, they can easily be made at the low cost to withstand several pounds of force from the spring 44 without being permanently deformed Moreover, with both the button 36 and support cup 30 being themselves conducting and electrically connected to each other in a positive way by the spring 44, it is not necessary to provide a separate connection of the wire 52 to the upper end cap 18, as is the case for prior fuses of the type described. for example, in the above-cited Brandt et al. patent.

A second embodiment of the invention is a fuse generally as in FIG. 1, but in which an indicating mechanism 56 is shown in FIGS, 3 and 4. Like reference numerals are used in FIGS. 3 and 4. The mechanism 56 includes a metal upper end cap 58 having a support cup 60 integrally formed in it with three convex projections 62 in the wall portion of the cup 60. The bottom of the cup 60 has an opening for fitting a bushing 64. An indicating button 66, spring 68, restraining wire 70, and bead 72 are assembled generally as were those corresponding parts in the first embodiment. The operation is similar to that of the first embodiment. A desirable feature of the second embodiment is that the assembly steps of brazing a support cup to an upper end cap and then fastening a washer to it to form a bottom, as necessary for the first embodiment, are eliminated.

GENERAL CONSIDERATIONS The bead provided in both embodiments above for holding the restraining wire under tension against the spring force of several pounds has proved particularly effective for this purpose. The previous approach of wedging pins in the apertures and applying epoxy or other settable adhesive has not been found adequate for holding the wire against such a relatively strong force. The bead may, of course, be of metal, glass. or other suitable material rigid enough to hold the wire under tension and sufficiently stable.

While the spring forces in the mechanisms of the preferred embodiments were about 2 pounds, it is considered desirable that they be closer to 5 pounds, so that the force exerted by the button will be sufficient to activate most any type of commonly used protective circuit interrupting device, such as a breaker or relay. Upon increasing the spring force, it may be necessary to increase the breaking strength of the restraining wire. This may be achieved, for instance, by simultaneously increasing the diameter and the resistivity or by utilizing a different material altogether. In general, it is desirable that the spring force by on the order of several pounds, or at least 2 pounds.

What is claimed is:

1. An electric fuse of the type having an insulating housing, electrically conductive terminal caps at opposite ends of the housing, at least one main fusible element inside the housing connecting together electrically the terminal caps, and an indicating mechanism at one of said terminal caps for indicating fusing of the main element, wherein the improvement comprises said indicating mechanism comprising:

an electrically conductive support cup having its rim aligned with, and adjacent, an opening in one of said terminal caps and rigidly fastened in position to said cap in electrical connection thereto;

an apertured insulating bushing plug fitted into an opening in the bottom of said support cup;

a cup-shaped electrically conductive indicator button telescopingly disposed inside said support cup with its bottom remote from the bottom of said support cup;

a spring inside said button and support cup exerting between said button and said support cup a biasing force for moving said button away from the bottom of said support cup, and

an electrical resistance restraining wire passing through said aperture in said bushing plug to the inside of said support cup and being secured to the bottom of said button, said wire being also secured under tension at said bushing plug, so that said button is held by said wire against the biasing force of said spring, the portion of said wire on the outside of said support cup passing through the interior of said insulating housing of said fuse and being electrically connected to the other of said terminal caps. 2. The fuse defined in claim 1 wherein said bushing plug is provided with two apertures, a length of said wire emerging from each of said apertures on the side of the plug remote from said button and passing in opposing directions through an opening in a bead resting on said plug between said apertures, the opening in said bead being along a direction substantially parallel to the surface of said plug, said wire being interlocked adjacent said bead after passing through said opening.

3. The fuse defined in claim 2 wherein said spring exerts a force between said button and said support cup on the order of about several pounds.

4. The fuse defined in claim 3 wherein said spring is electrically conductive and contacts directly said button and the bottom of said support cup to electrically connect them together. 

1. An electric fuse of the type having an insulating housing, electrically conductive terminal caps at opposite ends of the housing, at least one main fusible element inside the housing connecting together electrically the terminal caps, and an indicating mechanism at one of said terminal caps for indicating fusing of the main element, wherein the improvement comprises said indicating mechanism comprising: an electrically conductive support cup having its rim aligned with, and adjacent, an opening in one of said terminal caps and rigidly fastened in position to said cap in electrical connection thereto; an apertured insulating bushing plug fitted into an opening in the bottom of said support cup; a cup-shaped electrically conductive indicator button telescopingly disposed inside said support cup with its bottom remote from the bottom of said support cup; a spring inside said button and support cup exerting between said button and said support cup a biasing force for moving said button away from the bottom of said support cup, and an electrical resistance restraining wire passing through said aperture in said bushing plug to the inside of said support cup and being secured to the bottom of said button, said wire being also secured under tension at said bushing plug, so that said button is held by said wire against the biasing force of said spring, the portion of said wire on the outside of said support cup passing through the interior of said insulating housing of said fuse and being electrically connected to the other of said terminal caps.
 2. The fuse defined in claim 1 wherein said bushing plug is provided with two apertures, a length of said wire emerging from each of said apertures on the side of the plug remote from said button and passing in opposing directions through an opening in a bead resting on said plug between said apertures, the opening in said bead being along a direction substantially parallel to the surface oF said plug, said wire being interlocked adjacent said bead after passing through said opening.
 3. The fuse defined in claim 2 wherein said spring exerts a force between said button and said support cup on the order of about several pounds.
 4. The fuse defined in claim 3 wherein said spring is electrically conductive and contacts directly said button and the bottom of said support cup to electrically connect them together. 